1. Field of the Invention
The present invention relates generally to video receiver monitors and more particularly to a system for determining the channel to which a video wave receiver is tuned by the injection of signals at the antenna input of the video receiver.
2. Description of the Prior Art
Various arrangements of the prior art have been proposed to determine the channel to which a television receiver is tuned. For example, mechanical arrangements for determining the channel to which a television receiver is tuned and requiring direct mechanical couplings to the channel selection elements of a receiver tuner are disclosed in U.S. Pat. Nos. 2,751,449 which issued to Krahulec, et al. on June 19, 1956 and 4,038,504 which issued to McAnulty, et al. on July 26, 1977. While such mechanical interconnection arrangements are reliable, the direct mechanical couplings require skilled installation and removal of parts. Further, variations in the mechanical configurations of tuner elements of various receivers require a large assortment of adapters to implement the mechanical couplings.
U.S. Pat. No. 3,973,206 which issued to D. E. Haselwood, et al. on Aug. 3, 1976 discloses a method of determining channel tuning by monitoring the varactor diode tuning voltage to overcome the problems of mechanical couplings. However, installation of this type of monitoring arrangement requires internal connections in the TV receiver and is applicable only to voltage tuned receivers.
Another method to determine channel tuning disclosed in U.S. Pat. No. 2,864,941 which issued to Currey, et al. on Dec. 16, 1958 detects the effect of switching tuned circuits coupled to the antenna input in synchronism with the horizontal sweep. This method requires a transmission line that is reasonably well matched to the receiver for proper functioning.
Another method to determine the channel to which a receiver is tuned is disclosed in U.S. Pat. No. 4,044,376 which issued to J. L. Porter on Aug. 23, 1977 and U.S. Pat. No. 4,058,829 which issued to J. R. Thomson on Nov. 15, 1977. In this method, the antenna input to the receiver is switched to an RF oscillator during an RF pulse to eliminate the possibility of signals from the antenna causing video outputs that might be confused with the RF oscillator pulse input. The RF oscillator is pulsed in synchronism with the sweep of the receiver and stepped in frequency to each possible receivable channel. When the video signal shows a pulse in correspondence with the RF oscillator, the RF oscillator is assumed to be set to the channel tuned by the receiver. This method has the disadvantage that false indications can occur should a receiver be tuned to a UHF channel such that harmonics of a lower frequency VHF channel would fall into the UHF channel. For example, VHF channel 2 has a ninth harmonic falling into UHF channel 20. Furthermore, this method determines the channel tuned by the receiver and not the channel being received since the method does not in any way depend on the reception of a transmitted video carrier channel signal.
Another method disclosed in U.S. Pat. No. 2,903,508 which issued to J. L. Hathaway on Sept. 8, 1959 adds single sideband audio signals to an NTSC video signal at a low level and in between the spectral peaks of horizontal sync harmonics to allow a usable audio signal to be recovered without noticeably degrading the video to the viewer. This method when applied to the determination of a channel tuned by a receiver includes the disadvantage of the necessity to accurately control the level of the coupled signal to prevent too large a signal from causing interference in the viewed picture while providing a large enough signal such that a reliable indication of the signal coupled into the receiver is sufficiently above the noise due to video signals.